1. Field of the Invention
The present invention relates to novel polyamide-imide resins and processes for the production thereof, and more particularly, to melt moldable aliphatic and aromatic polyamide-imide copolymers having high glass transition temperatures and good heat resistance and processes for the production thereof.
2. Description of the Related Art
In general, aromatic polyamide-imide resins have excellent mechanical characteristics and heat resistance. The aromatic polyamide-imide resins heretofore proposed are thermally unstable upon melt molding due to the small difference between the flow initiating temperature and the thermal decomposition initiating temperature, and further the fluidity when melted is so low that melt molding is difficult.
In order to solve such problems of aromatic polyamide-imide resins. Japanese Patent Publication No. 39616/1988 proposes preparing polyamide-imide copolymers by using diamines comprising 4 benzene nuclei or more connected through ether bonds so as to improve the moldability.
However, the starting material, i.e. particular aromatic diamines having 4 benzene nuclei or more, can not be commercially produced with ease. In addition, direct polymerization for producing such polymers requires a long reaction time at an elevated temperature and thereby, the industrial production is problematic. In view of the foregoing, the proposed wholly aromatic polyamide-imide polymers are not always satisfactory upon manufacturing commercially from a practical point of view.
On the other hand, polyamide-imide resins having an amine skeleton of aliphatic nature have a low glass transition temperature and the heat resistance is not satisfactory.
Further, U.S. Pat. No. 3,939,029 discloses that trimellitoyl chloride anhydride is reacted with an aliphatic diamine to prepare a polyamide acid, which is then heated and dehydrated to form an aliphatic, aromatic polyamide-imide resin. However, the reactivity of the aliphatic diamine with trimellitoyl chloride anhydride is so low that the product has only a low molecular weight, and thereby the product can be used as adhesives. However, the product does not have a molecular weight sufficiently high to produce molded articles.
U.S. Pat. Nos. 4,348,513 and 4,291,149 disclose a process for producing injection moldable polyamide-imide resins where tricarboxylic acid anhydride groups constituting the polyamide-imide polymer are regularly arranged such that the head to head and tail to tail are linked sequentially. There are proposed aliphatic aromatic polyamide-imide resins produced by the reaction of fully or partly acylated diamines, tricarboxylic acid anhydrides and diamines. Since free carboxylic acids and amines can not be fundamentally reacted with each other, an acylating agent such as acetic anhydride is used to effect the amidation. In the condensation of amidation, acetic acid and the like are formed and in the condensation of imidization, water is formed. It is necessary to remove the acetic acid, water and the like thus formed at a high temperature under reduced pressure. Since the process involves acylation, the operation becomes complicated.
For the purposes of solving drawbacks of the prior art, the present inventors have conducted research and found that polyamide-imide resins capable of at least partly solving the drawbacks, for example, the resins of higher heat resistance, can be obtained by blocking the ends of the polymer molecule with particular end blocking groups and further said polyamide-imide resin can be efficiently produced by reacting aromatic tricarboxylic acid anhydrides, diisocyanates, diamines and end blocking agents in the presence of alkali metal compounds as a catalyst in a dipolar aprotic solvent.